The present invention relates to geometrical measuring technology, and a device according to the present invention serves for the indirect determination of the position of points with the help of a tracker, wherein there is no need for a direct line of sight between the point position to be determined and the tracker.
A tracker includes means for generating a light beam, in particular a laser beam. With the help of a mirror, the laser beam is directed to a target point, from where it is reflected back into the tracker by a retroreflector. The tracker further includes means for automatically aligning the mirror such that the path of the reflected beam, if at all possible, is the same as the path of the emitted beam. These means allow the tracker to follow a moving target point with the mirror or with the laser beam respectively. The orientation of the mirror is recorded as a measured value representing the direction from the tracker to the target point. The tracker even further includes an interferometer for recording distance changes between the tracker and a moving target point (relative distance measurement). If so required, the tracker also comprises means for an absolute distance measurement, which means are, for example, based on the Principle of Fizeau.
Trackers are primarily used for recording the track of a moving target point or for scanning surfaces, wherein a target point is moved over the surface. However, trackers can also be used to determine the position of stationary target points, wherein these positions are defined by the (known) tracker position, by the orientation of the tracker mirror (direction of the tracker beam), and by the absolute distance between tracker and target point. For direct measurements of this kind, a direct sight connection between the target point and the tracker is needed, i.e., the tracker beam must not be interrupted between the tracker and the target point.
It would be desirable when using a tracker for determining point positions or track points to be able to do so without direct line of sight connection from the tracker to the point or track. If this were possible, the installation of a multitude of trackers or the displacement of a single tracker could be avoided in many instances.
For determining point positions using a theodolite or a digital camera (photogrammetry), tools for indirect measurement are used if there is no direct line of sight connection from the measuring instrument to the point. Such a tool comprises a reference point and at least two target points having exactly defined positions relative to each other. The tool is designed to be positioned such that the position of the reference point relative to a point position to be determined is exactly defined. For determining the position of a specific point, the tool is positioned such that the named relationship between the reference point and the specific point is established and such that between the target points of the tool and the measuring instrument there is a direct line of sight connection. Thereupon, the positions of the target points are determined and from the known positions of the target points relative to one another the position and at least partially the orientation of the tool are determined. From the calculated position and orientation of the tool, the position of the reference pointxe2x80x94and from the known relative positions of reference point and sought point, the position of the sought point is calculated. It is also known to use such a correspondingly equipped tool for determining not only a point position but also the orientation of the surface, on which this point is located.
The relative positions of the reference point and the point to be determined are defined purely mechanically, for example with a feeler, a tip or a pin with shoulder, or else opto-mechanically, for example through a virtual point, a reticule plate or cross-hairs. The arrangement of the target points usually represents a simple geometrical shape (straight line, circle, sphere), because such a shape can be mathematically described in a very simple manner. Using the high data processing capacities available today more complicated arrangements are also applicable.
Tools for determining point positions, to which no direct line of sight connection exists, and/or for determining surface orientations are described, for example, in the publications SE-456454 or WO-91/16598.
Publication DE-4038521 also describes tools of the described type, which tools comprise two retro-reflectors positioned together with the reference point in one straight line. The distances and directions of the two retro-reflectors from the measuring instrument (tachymeter) are measured after each other. In the same publication it is suggested also to replace the two retro-reflectors by only one reflector and to displace this one reflector from a first into a second position between the two successive measurements.
The tools for the indirect determination of point positions and/or of surface orientations as described above can be used together with a tracker, for example, for determining the position of a point from which there is no direct line of sight connection to the tracker. For this purpose, the target points of the tool have to be designed as retro-reflectors. For measuring, the tracker is directed at one of the target points of the tool, the direction and the absolute distance from the tracker to the aimed at target point is determined, and this procedure is repeated for all target points of the tool. The measured directions and distances are then processed in known manner.
This procedure is possible, but rather elaborate, because the tracker has to be aimed at every single target point and this has to be done by hand unless there are special aids installed, e.g. a digital camera functionally connected with the tracker or means for enlarging the field of vision of the tracker. Furthermore, for the measurement of every target point an absolute distance measurement has to be carried out, a measurement, which due to the means as available today is substantially less accurate than the interferometric, relative distance measurement.
The elaborate procedure as described above can be avoided if the target points are arranged on the tool so close together that all of them appear in the very narrow field of view of the tracker without changing the tracker direction. This, on the other hand, restricts the accuracy considerably.
It is an object of the invention to create a device applicable for measurements using a tracker and, like the known tools mentioned above, in particular serving the determination of positions of points without a direct line of sight connection to the tracker, and/or for determining surface orientations, wherein by using the inventive device and a tracker, the disadvantages occurring when using the known tools and a tracker are avoided. The device according to the invention makes it possible to carry out indirect measurements with the help of a tracker easily and with a high accuracy, such as is customary for other tracker measurements.
It goes without saying that the device according to the invention can also be used for determining the position of points, for which, while there is a direct line of sight connection to the tracker, an indirect measurement is advantageous for other reasons. A reason of this type exists, for example, for points on a surface, on which no retro-reflector can be arranged such that its optical center is located in the point position to be determined. In such a case, the position determined by direct measurement needs to be corrected, the correction being dependent not only on the known size of the retro-reflector (distance between the point to be measured and the optical centre of the reflector), but also on the direction of this distance which may not be determined easily.
Instead of the at least two target points and their defined spatial relationship to the reference point of known tools for the indirect determination of point positions and/or surface orientations, the measuring device according to the invention only comprises one target point in the form of a retro-reflector, wherein this single target point is movable along a trajectory path, the position of the trajectory path relative to the reference point being exactly defined. For determining the position and orientation of the device, the tracker is aimed at the target point, the target point is moved along the path such that the tracker is able to follow it, and the path covered by the target point is recorded or predefined positions (e.g., starting position and end position) of the target point moving along the path are recorded. From the recorded data, the position and orientation of the device is computed in a known manner and therefrom the position of the reference point and further the sought point position and/or the surface orientation are calculated.
Using the described procedure, aiming the tracker beam at a target point and absolute distance measurement are necessary solely for the starting position of the target point. For the rest of the measuring procedure, the tracker follows the moving target point and only distance variations are measured (interferometric, relative distance measurements), which can be carried out with very high accuracy. This means that determination of the orientation of the device is based on relative distance measurements only, while determination of the position of the device is based on one absolute distance measurement. The accuracy achieved is higher than the accuracy of a method in which the tracker is to be aimed at at least two target points of a tool and the positions of the target points each have to be determined by means of an absolute distance measurement.
The absolute distance to the sole target point of the measuring device according to the invention can be determined with a quasi-absolute measurement, i.e., in that from a reference position being xe2x80x9cvisiblexe2x80x9d from the tracker and having a predefined direction and distance from the tracker, the instrument is moved into the position to be measured and is tracked by the tracker during this movement.
The trajectory path of the target point on the measuring device according to the invention advantageously has a shape, which can be mathematically described in a simple manner, for example it is a straight line or a circular arc.
In the same manner as for measuring using the at least two target points of a known tool for indirect measurement, it is necessary also for measuring with the device according to the invention, that the device is not moved during measuring. In order to reduce or the prevent forces acting on the measuring device as a result of the movement of the target point along its trajectory and such prevent movements of the device relative to the point to be measured produced by such forces, it is advantageous to install the measuring device fixedly and to install the drive for moving the target point on the device itself.
It is possible to build a control mechanism into the calculation for the determination of the position and orientation of the measuring device. If a trajectory path of the target point as recorded by the tracker does not correspond to the defined path of the target point on the measuring device to within a predefined tolerance range, then this is interpreted as a movement of the device during measurement and the measurement is rejected as unusable.